This invention generally relates to circuit materials, methods for the manufacture of the circuit materials, and articles formed therefrom, including circuits and circuit laminates.
As used herein, a circuit material is an article used in the manufacture of circuits and multi-layer circuits, and includes circuit subassemblies, bond plies, resin coated conductive layers, unclad dielectric layers, and cover films. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric layer. Double clad circuit laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more circuits together using bond plies, by building up additional layers with resin coated conductive layers that are subsequently etched, or by building up additional layers by adding unclad dielectric layers followed by additive metallization. After forming the multilayer circuit, known hole-forming and plating technologies can be used to produce useful electrical pathways between conductive layers.
Mineral and ceramic particulate fillers are widely used to control the dielectric and physical properties of polymeric dielectric composite materials used in dielectric layers. Particularly where a low dielectric constant is desired, hollow glass or ceramic microspheres can be used. For example, in U.S. Pat. No. 4,134,848, Adicoff et al. describe a composite for a stripline board material having hollow, air-filled glass microspheres in a hydrocarbon matrix. Okada and Fujino, in U.S. Pat. No. 4,661,301, teach the production of a hollow glass microsphere-filled polymer composite made by directly extruding the molten composition into the opening of a vertical double belt press. In U.S. Pat. No. 5,126,192, Chellis et al. disclose a filled prepreg material having a dielectric constant below 3.2, made with very small experimental hollow microspheres from 3M, and hollow glass microspheres from Grace Syntactic of Canton, Mass. U.S. Pat. No. 4,610,495 to Landi discloses the use of a layer of elastomer filled with hollow spheres for controlling impedance in a solderless connector for a microelectronic device. U.S. Pat. No. 4,994,316 to Browne and Jarvis disclose a bonding layer for circuit boards containing hollow glass microspheres.
The manufacture of small glass or ceramic microspheres (diameters=1 to 1000 micrometers) is costly, and therefore results in significantly more expensive circuit materials. While the synthetic microspheres described above are used for purposes of improving electrical properties in dielectric circuit substrates, there remains a need in the art for low dielectric constant, low loss circuit materials that are suitable for use in demanding applications such as high frequency applications.